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Background: Controversies in nomenclature, especially of hearts with "single ventricle," have clouded discussion and understanding of the anatomy. Many patients with such malformations are submitted to the Fontan procedure as definitive surgical palliation. The spectrum of anatomy among these patients is wide and deserves analysis in an effort to provide a simple framework for description and to eliminate confusion., Methods: We reviewed 138 successive patients undergoing the Fontan operation at one institution to demonstrate the variability in pathology., Results: Only 89 patients (65%) had a univentricular type of atrioventricular connection. All but 5 patients had two ventricular chambers. Among the 49 patients with biventricular atrioventricular connections, 43 had a hypoplastic ventricle that precluded a biventricular repair., Conclusions: Full understanding of the malformations that may preclude a "biventricular" repair and hence necessitate a Fontan procedure requires knowledge of the different forms of univentricular atrioventricular connection that, although usually associated with two ventricular chambers, are seldom amenable to a "two-pump repair." Understanding of those types of "biventricular heart" that preclude a two-pump repair (eg, severe hypoplasia of the left ventricle or the right ventricle) or are associated with high risks (eg, straddling atrioventricular valve) is also important.

Most hearts described as "single ventricle" or "univentricular heart" possess 2 ventricular chambers, even though 1 is usually described as an "outlet chamber." This stems from the wide acceptance that the criterion of a single ventricle is the presence of a double-inlet atrioventricular (AV) connection. In recent years, using this criterion, an attempt was made to show how hearts with double-inlet right ventricle or "classic tricuspid atresia" were (in terms of ventricular morphology) just as univentricular as "single ventricle with outlet chamber." This attempt brought still further confusion to an already contentious topic. The root of the problem clearly is the injudicious use of the adjective "single" or "univentricular." Conventionally it is used to describe the ventricular mass. In most hearts with double-inlet connection it is not the ventricles that are univentricular; it is the AV connection. The concept of a univentricular AV connection, then, appropriately groups hearts with double-inlet along with those having absence of 1 AV connection. It distinguishes this entire group from those other hearts with biventricular AV connections (each atrium connected to its own ventricle). The term "univentricular AV connection" is thus a collective one for all those hearts in which the atria connect to only 1 ventricle. Confusion will be completely removed if individual hearts are described for what they are in terms of AV connection and ventricular morphology (for example, double-inlet left ventricle with rudimentary right ventricle rather than single ventricle with outlet chamber).

Two cases are described of a most unusual variant of two-chambered right ventricle. In both the ventricular septal defect was between the distal chamber of the right ventricle and the left ventricle. However the extensive dividing 'septum' between proximal and distal parts of the right ventricle converted the latter, haemodynamically, into part of the left ventricle. In the first case the distal chamber supported the aorta in the left anterior position, the pulmonary artery arising from the proximal part of the right ventricle. In the second the pulmonary artery arose from the distal chamber and the aorta from the proximal chamber. Though in both the ventriculoarterial connection was double outlet right ventricle, functionally there was arterial concordance in case 1 and discordance in case 2. A further disconcerting feature was the resemblance of the distal right ventricular chamber to the rudimentary chamber of a univentricular heart of left ventricular type.

Four patients are described with a recently recognized variant of double outlet right ventricle. Clinical examination favoured tetralogy of Fallot, but the chest X-ray suggested corrected transposition. Catheterization and angiocardiography showed that the aorta was to the left of the main pulmonary artery, and both arose from a normally positioned morphological right ventricle. Egress of blood from the left ventricle was through a subaortic ventricular septal defect. In all patients severe pulmonary stenosis was present and the right coronary artery ran an anomalous course anterior to the pulmonary valve ring. Two children had successful total correction, and one a palliative Blalock-Taussing shunt. Necropsy material from the fourth patient allowed confirmation of the ventricular morphology and the conducting tissued was examined. In corrective surgery, blood from the left ventricle was rerouted into the aorta by an intraventricular baffle. Pulmonary stenosis was relived by infundibulectomy and outflow tract patch.

We have studied the disposition of the cardiac conducting tissues in four hearts from situs solitus individuals possessing primitive ventricles without outlet chambers. These hearts correspond to the type of univentricular heart defined as common ventricle by Lev. All the hearts studied possessed normally positioned great arteries. Two groups, each consisting of two hearts, could be distinguished. The first type possessed a small posterior ridge which divided the posterior portion of the primitive ventricle into right and left ventricular sinuses. The papillary muscles to the atrioventricular valves were separate structures and arose on each side of this posterior ridge. The conducting tissues in these hearts arose from an atrioventricular node situated in the atrial septum but deviated posteriorly. The atrioventricular bundle pierced the fibrous annulus posteriorly and descended on the posterior ridge, lying to its left side. A bifurcation was not identified, and bundle branches were not present. The other two hearts had no posterior ridge. A common posterior papillary muscle supported both atrioventricular valves, and in one a marked anterior muscle bar produced obstruction of the pulmonary outflow tract. The connecting atrioventricular node was situated laterally in the right atrioventricular valve orifice, and the atrioventricular bundle descended into the right parietal wall of the primitive ventricle. A bifurcation and bundle branches were not observed. The disposition of conducting tissue in these hearts differs from that found in "primitive ventricle with outlet chamber" in that the connecting atrioventricular node and bundle are situated anteriorly and are intimately related to the transposed pulmonary artery outflow tract in the latter anomaly. The surgical significance of these findings is emphasized.

The ventricle morphology is described in hearts in which the entire atrial inputs are committed to a single chamber in the ventricular mass, this being considered the criterion for definition of the univentricular heart. Such a definition does not exclude from the univentricular category hearts with two chambers in their ventricular mass providing that the second chamber does not receive a direct atrial input. Three types of ventricular morphology were identified: hearts with a main chamber of left ventricular pattern and a rudimentary chamber of right ventricular pattern; hearts with a main chamber of right ventricular pattern and a rudimentary chamber of left ventricular pattern and hearts with a sole chamber in their ventricular mass of indeterminate pattern. Hearts may also exist of right or left ventricular type without rudimentary chambers. Variation was noted in type and position of the rudimentary chambers. Chambers of either right or left ventricular type were found supporting either or both great arteries (outlet chambers), and also without a direct outlet (trabecular pouches). Right ventricular rudimentary chambers were usually anterior but could be right or left sided; left ventricular chambers were posterior but again could be right or left sided. Hearts with these ventricular morphologies possessed either double inlet atrioventricular connexions, or absence of the right or left atrioventriuclar connexion. In double inlet ventricles, cases were observed with two valves, a common valve, one imperforate valve, and with straddling atrioventricular valves. A straddling valve was also found in hearts with absence of one atrioventricular connexion. Further variation was found in the pattern of arterial connexions. It is concluded that ventricular morphology is an important feature of the iniventricular heart but must be considered in the context of other important and variable features.

Seventeen cases are described in which both atria connect directly to a chamber with right ventricular characteristics. The atria connected through separate atrioventricular valves in six hearts and a common valve in 11. All hearts had a posterior rudimentary chamber. The septum which separated it from the main chamber was directed to the crux of the heart. Ten hearts were from patients with atrial situs solitus and seven from patients with atrial situs ambigous. Arterial connections were concordant in three cases, had a double outlet from the main ventricular chamber in nine and single outlet of the heart in five. The patent artery always arose from the main chamber, with pulmonary atresia in three and aortic atresia in two. This and other studies indicate that double inlet atrioventricular connection does not predict the morphology of the main chamber. Although usually associated with a main chamber of left ventricular type, it may also be associated with a main chamber having right ventricular characteristics. Both types should be considered as univentricular hearts; the posterior chamber in hearts of right ventricular type are analogous to the anterior chamber in univentricular hearts of left ventricular type and are a rudimentary chamber rather than a hypoplastic ventricle. In the right ventricular form of univentricular heart, the trabecular zone of the rudimentary chamber is of left ventricular type.

Venous correction for complete transposition with intact ventricular septum remains the preferred surgical option in most centers (Mustard or Senning procedures). The long-term function of the morphologically right ventricle and tricuspid valve, which continue to sustain the systemic circulation thereafter, remains a matter of concern. Some centers have been performing arterial redirection ("arterial switch") as the surgical treatment of transposition. Consequently, the status of the left ventricle and mitral valve is of equal interest. Yet, detailed morphological studies of the ventricles, atrioventricular valves, and their tension apparatus in "simple" complete transposition are lacking. We have examined the morphology of the left ventricle and the spatial relationships of the component parts of the interventricular septum (part I) as well as the morphology of the right ventricle and the tricuspid valve (part II). A total of 77 specimens with complete transposition and intact ventricular septum were examined and compared with 50 normal hearts. Consistent deformities were noted in relation to the orientation of the ventricular septum. The ratios of ventricular outlet to the inlet lengths were increased in both ventricles and the posterior arterial valve was abnormally related to the atrioventricular valves. These anomalies are similar in some respects to those found in atrioventricular septal defects. Their significance in relation to long-term function remains unclear.

Two main theories exist for the explanation of univentricular hearts. One states that the bulboventricular septum becomes realigned to form the interventricular septum, and that univentricular hearts are a consequence of failure of this realignment. The other states that bulboventricular and interventricular septa are different structures, and that the univentricular heart results from failure of formation of the posterior interventricular septum. Four hearts are described in which both the posterior septum and an anterior bulboventricular septum are present. In each heart, therefore, the right ventricular sinus is separated both from the left ventricular sinus and from a discrete outlet chamber which supports the pulmonary artery. It is argued that these findings militate strongly against theories proposing reorientation of the bulboventricular septum to form the definitive interventricular septum. They support strongly the concept that the definitive right ventricle is formed in part from the bulbus and in part from the primitive ventricle. On the basis of these findings, it is suggested that the distinctive feature of the univentricular heart is absence of the posterior septum. Such hearts can properly be termed 'primitive ventricle'. It is also suggested that hearts with atretic or straddling valves should be included within this category.